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1. EP2430646 - ELECTROSTATIC ION TRAP

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Claims

1. An ion trap (110) comprising:

an electrode structure, including first and second opposed mirror electrodes (6, 7) and a central lens (3) therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic;

characterized by an AC excitation source (21) having an excitation frequency f that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation source (21) being connected to the central lens (3).


  2. The ion trap (110) of Claim 1, further including a scan control (100) that mass selectively reduces a frequency difference between the AC excitation frequency and about twice the natural oscillation frequency of the ions.
  3. The ion trap (110) of Claim 2, wherein the scan control (100) sweeps the AC excitation frequency f at a sweep rate in a direction from an excitation frequency higher than twice the natural oscillation frequency of the ions to achieve autoresonance.
  4. The ion trap (110) of Claim 2, wherein the scan control (100) sweeps the AC excitation frequency f at a sweep rate in a direction from an excitation frequency lower than twice the natural oscillation frequency of the ions.
  5. The ion trap (110) of Claim 2, wherein the scan control (100) sweeps the AC excitation frequency f at a sweep rate set such that d(1/ f n)/dt is about equal to a constant and n is greater than zero.
  6. The ion trap (110) of Claim 5, wherein n is approximately equal to 1.
  7. The ion trap (110) of any of the preceding claims, wherein the first opposed mirror electrode (6) of the electrode structure includes

a) a first plate-shaped electrode (1) with at least one aperture, located off-axis with respect to an axis of the opposed mirror electrode structure; and

b) a second electrode (6) shaped in the form of a cup, open towards the central lens (3), with a centrally located aperture; and

the second opposed mirror electrode (7) of the electrode structure includes

i) a first plate-shaped electrode (2) with an axially located aperture; and

ii) a second electrode (7) shaped in the form of a cup, open towards the central lens (3), with a centrally located aperture; and

the central lens (3) is plate-shaped and includes an axially located aperture.
  8. The ion trap (110) of any of the preceding claims, configured as a mass spectrometer, further including an ion source (16) that includes at least one electron emissive source that creates ions by electron impact ionization of a gaseous species, and an ion detector (17).
  9. The ion trap (110) of Claim 8, wherein the at least one electron emissive source is a hot filament located off-axis relative to the electrode structure, and further including a repeller (85) located behind the hot filament that focuses electrons, or the at least one electron emissive source is a cold electron emissive source
  10. The ion trap (110) of Claim 8, wherein the ion detector (17) is a charge-sensitive transimpedance amplifier.
  11. The ion trap (110) of Claim 8, wherein the ion detector (17) detects ions by measuring the amount of RF power absorbed from the AC excitation source (21) as the AC excitation source (21) frequency varies, or the ion detector (17) detects ions by measuring the change in electrical impedance of the electrode structure as the AC excitation frequency varies, or the ion detector (17) detects ions by measuring the current induced by image charges as the AC excitation frequency varies, or the ion detector (17) detects ions by measuring the amount of RF power absorbed from the AC excitation source (21) as the magnitude of the electrostatic potential varies, or the ion detector (17) detects ions by measuring the change in electrical impedance of the electrode structure as the magnitude of the electrostatic potential varies, or the ion detector (17) detects ions by measuring the current induced by image charges as the magnitude of the electrostatic potential varies.
  12. A method of trapping ions in an ion trap (110) comprising:

producing an anharmonic electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, in an electrode structure that includes first and second opposed mirror electrodes (6, 7) and a central lens (3) therebetween;

characterized by exciting confined ions at a frequency of about twice the natural oscillation frequency of the ions with an AC excitation source (21) having an excitation frequency f, the AC excitation source (21) being connected to the central lens (3).


  13. The method of Claim 12, further including the step of scanning the excitation frequency of the AC excitation source (21) and mass selectively reducing a frequency difference between the AC excitation frequency and about twice the natural oscillation frequency of the ions.
  14. The method of Claim 13, wherein scanning the excitation frequency is performed at a sweep rate from an excitation frequency higher than about twice the natural oscillation frequency of the ions, to mass selectively achieve autoresonance as the frequency difference approaches zero.
  15. The method of Claim 13, wherein scanning the excitation frequency is performed at a sweep rate set such that d(1/ f n)/dt is about equal to a constant and n is greater than zero.
  16. The method of Claim 15, wherein n is approximately equal to 1.
  17. The method of Claim 12, wherein the first opposed mirror electrode structure (6) includes a first plate-shaped electrode (1) with at least one aperture, located off-axis with respect to an axis of the opposed mirror electrode structure and a second electrode (6) shaped in the form of a cup, open towards the central lens (3), with a centrally located aperture, and the second opposed mirror electrode structure (7) includes a first plate-shaped electrode (2) with an axially located aperture and a second electrode (7) shaped in the form of a cup, open toward the central lens (3), with a centrally located aperture, and the central lens (3) is plate-shaped and includes an axially located aperture.
  18. The method of any one of Claims 12-17, further including using an ion source (16) that includes at least one electron emissive source that creates ions by electron impact ionization of a gaseous species, and an ion detector (17), configured as a mass spectrometer.
  19. The method of Claim 18, wherein the at least one electron emissive source is a hot filament located off-axis relative to the electrode structure, and further including a repeller (85) located behind the hot filament that focuses electrons, or the at least one electron emissive source is a cold electron emissive source.
  20. The method of Claim 18, wherein the ion detector (17) is a charge-sensitive transimpedance amplifier.
  21. The method of Claim 18, wherein the ion detector (17) detects ions by measuring the amount of RF power absorbed from the AC excitation source (21) as the AC excitation source (21) frequency varies, or the ion detector (17) detects ions by measuring the change in electrical impedance of the electrode structure as the AC excitation frequency varies, or the ion detector (17) detects ions by measuring the current induced by image charges as the AC excitation frequency varies, or the ion detector (17) detects ions by measuring the amount of RF power absorbed from the AC excitation source (21) as the magnitude of the electrostatic potential varies, or the ion detector (17) detects ions by measuring the change in electrical impedance of the electrode structure as the magnitude of the electrostatic potential varies, or the ion detector (17) detects ions by measuring the current induced by image charges as the magnitude of the electrostatic potential varies.